As mobile phones become more complicated and able to support many different radio standards such as Global System for Mobile Communications (GSM), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access 2000 (CDMA2000) and Time Division Synchronous Code Division Multiple Access (TD-SCMA) etc, the need for highly-adaptive channel filtering in the digital domain becomes ever stronger. The radio receivers of mobile phones already contain significant amounts of digital signal processing. Most of the digital filters used are based on finite impulse response (FIR) or infinite impulse response (IIR) blocks which process the wanted signal in the time domain. This is adequate in terms of power efficiency and area occupancy, but it tends to have serious short-comings in terms of ease of design and adaptability.
Carrying out the digital filtering in the frequency domain can offer equivalent performance but with significantly improved ease of design, adaptability and control. Digital filters offering selectivity on demand in response to changes in the signal-interference environment around the mobile phone would enable significant savings in battery consumption.
Highly-adaptive digital filtering in the frequency domain has already been proposed as an important innovation in the context of Software-defined Radio (SDR), described e.g. in “The Digital Front-End of Software Radio Terminals” by Tim Hentschel at al., IEEE Personal Communications, p. 6-12, August 1999. It offers greater inherent flexibility and intuitive control than conventional time-domain digital filtering regimes. Future multi-mode mobile phones will be a key beneficiary of such technique.
The requirement for a frequency translation arises mostly in a receiver configured for low-IF operation in support of narrow-band radio standards such as GSM and EDGE. Moving the IF (intermediate frequency) away from zero frequency tends to reduce the impact of 1/f noise and helps with the elimination of DC offsets. When the frequency translation takes place in the digital domain, the wanted signal can be moved back to zero prior to demodulation in the baseband modem. It is to be noted that it is important to be able to translate the frequency by an arbitrary (if still small) amount. It should also be noted that if implemented in the time domain, the frequency translation involves a repeated complex multiplication of the signal at the sampling rate, which can consume significant computational resources and battery power.
The multi-mode requirement of a mobile phone implies multiple bit rates, which again implies the requirement of multiple sampling rates for the wanted signal as it passes throughout the digital parts of the radio. However, it would be preferable to use a single sampling and clock frequency for all modes, irrespective of the required bit rates. This necessitates the use of fractional rate-matching (i.e. mostly decimation) at the output of the radio, so that data is delivered to the baseband modem with the correct time stamps.
Furthermore, in previous radio systems based on the use of a fixed array of decimation filters and a programmable channel filter in the time domain, de-rotation and sampling-rate changes were achieved using an interpolation scheme of one sort or another. These two operations can account for up to 30% of the power consumption of the digital receiver hardware. Accordingly, it would be advantageous to carry out these operations in the frequency domain for little or no extra power consumption.